The tests on the plastic collapse of a stiffened plate were conducted under a concentrated lateral load at center. The load-deflection curves have shown the distinct change in mode at the collapse load of a stiffened plate because of the plastic collapse of a stiffener. Hence the plastic method of design based on the simple plastic theory was realized by the tests to be applicable to the design of a stiffened plate subjected to lateral loads. The deflections of a stiffened plate in the plastic range were calculated with the consideration of the effects of membrane actions under simplified boundary conditions, and the theoretical analysis was confirmed by the test results.